It is well known that the performance of semiconductor lasers has been improved tremendously by introducing a quantum well structure into an active layer. The quantum well utilizes the quantum size effect produced by thinning the layer thickness to about a de Broglie wavelength, that is, below about 10 nm (100 .ANG.), and, when it is applied to a semiconductor laser, a single layer or periodic plural layers are introduced into an active layer. In such a quantum well laser, carriers are injected into the quantum well layer, and light generation and recombination occur, thereby producing a tremendously large optical gain relative to a prior art bulk crystal active layer. As a result, lowering of the threshold current, narrowing of spectral linewidth, and enhancement of dynamic characteristics are realized.
FIG. 11 is a diagram showing a structure of a main part of a quantum well semiconductor laser proposed at the beginning of development of a quantum well semiconductor laser. In the figure, reference numeral 110 designates a p type InP substrate. A p type lower cladding layer 120 is disposed on the substrate 110. A multi-quantum well (MQW) structure active layer 140 comprising a plurality of InGaAs well layers and a plurality of InP barrier layers is disposed on the lower cladding layer 120. An n type InP upper cladding layer 160 is disposed on the quantum well active layer 140. An InGaAsP contact layer 170 is disposed on the upper cladding layer 160. A p side electrode 180 is disposed on the rear surface of the substrate 110 and an n side electrode 190 is disposed on the contact layer 170, respectively.
FIGS. 13(a)-13(c) are diagrams for explaining the operation of the MQW semiconductor laser shown in FIG. 11 where FIG. 13(a) is an energy band diagram of the conduction band edge in the vicinity of the active layer, FIG. 13(b) is a diagram showing refractive index corresponding to FIG. 13(a), and FIG. 13(c) is a diagram showing the electric field distribution corresponding to FIG. 13(a). In these figures, the same reference numerals used in FIG. 11 designate the same or corresponding elements as those shown in FIG. 11. Reference numeral 141 designates an InGaAs well layer and reference numeral 142 designates an InP barrier layer. The thickness of the well layer 141 is, for example, about 10 nm (100 .ANG.), the thickness of the barrier layer 142 is, for example, 10 nm (100 .ANG.), and the thicknesses of the lower cladding layer 120 and the upper cladding layer 160 are each, for example, about 1.5 .mu.m.
The electrons injected from the cladding layer 160 into the active layer 140 recombine with holes in the well layers 141, thereby generating light. In the quantum well semiconductor laser shown in FIG. 11, because of the quantum size effect, the carriers injected into the thin well layers 141 exhibit quantum dynamic wave motion so that a quite large optical gain per unit injection current is generated relative to a semiconductor laser having a bulk crystal active layer about 70 to 100 nm (700 to 1000 .ANG.) thick. On the other hand, the threshold current of a laser is represented by the product of optical gain and light confinement. Accordingly, in order to lower the threshold current of a laser, it is important to increase optical gain per unit injection current as well as to increase light confinement.
In the MQW semiconductor laser shown in FIG. 11, while it is possible to increase optical gain per unit injection current as described above, it is impossible to increase light confinement for the following reason. Although the vicinity of the active layer of a quantum well semiconductor laser has a refractive index distribution, as shown in FIG. 13(b), because the active layer (well layer 141) is quite thin, about 5 nm (50 .ANG.), the light generated in the active layer cannot react to the difference in the refractive index between the active layer and the cladding layer. As a result, the electric field distribution is broadened, smoothly, in the layer direction and has only a limited maximum inside the active layer. This is a phenomenon that cannot be seen in a semiconductor laser having a bulk crystal active layer about 70 to 100 nm (700 to 1000 .ANG.) thick. Here, the light confinement is represented by the hatched portion of the electric field distribution in FIG. 13(c). Because the electric field distribution is broadened in the layer thickness direction, it is impossible to increase light confinement in the quantum well layer. Accordingly, in the laser structure of FIG. 11, it is difficult to lower the threshold current.
FIG. 12 is a diagram illustrating a quantum well semiconductor laser structure devised to solve the above-described problems. In the figure, the reference numerals used in FIG. 11 are used to designate the same or corresponding elements. A p type InGaAsP light confinement layer 130 is disposed between the lower cladding layer 120 and the quantum well active layer 140. An n type light confinement layer 150 is disposed between the quantum well active layer 140 and the upper cladding layer 160.
FIGS. 14(a)-14(c) are diagrams for explaining the operation of the MQW semiconductor laser shown in FIG. 12 where FIG. 14(a) is an energy band diagram showing the conduction band edge in the vicinity of the active layer, FIG. 14(b) is a diagram showing the refractive index distribution corresponding to FIG. 14(a), and FIG. 14(c) is a diagram showing the electric field distribution corresponding to FIG. 14(a). In the figures, the reference numerals used in FIG. 12 are used to designate the same or corresponding elements. Reference numeral 141 designates an InGaAs quantum well layer and reference numeral 143 designates an InGaAsP barrier layer.
Electrons injected into the cladding layer 160 diffuse into the light confinement layer 150, are input to the quantum well active layer 140, and recombine with holes in the well layer 141, thereby generating light. As in the MQW semiconductor laser shown in FIG. 11, a large optical gain is generated because of the quantum size effect. In addition, the light confinement is represented by the hatched portion of the electric field distribution and it can be significantly increased by optimum design of the refractive index distribution. Although the electric field distribution is broadened in the layer direction in the simple structure shown in FIG. 11 and it is quite difficult to improve the light confinement, in the laser structure of FIG. 12 it is possible to optimize the refractive index distribution and concentrate the electric field in the vicinity of the quantum well layer. As a result, the light confinement layer coefficient when the light confinement layer is introduced is about 3 to 5 times that when no light confinement layer is provided.
Examples of the calculated light confinement coefficient and electric field distribution in the MQW semiconductor laser of FIGS. 11 and 12 will be described with reference to FIG. 17(a). For the laser structure of FIG. 11, the light confinement coefficient of the well layer when the active layer 140 comprises three InGaAs well layers 8 nm (80 .ANG.) thick and two InP barrier layers 10 nm (100 .ANG.) thick laminated alternatingly, as shown in FIG. 17(a), is 0.5%. The electric field distribution has a peak in the active layer and a smooth configuration, as shown in FIG. 17(b). On the other hand, for the laser structure of FIG. 12, the light confinement coefficient when the active layer 140 comprises three InGaAs well layers 6 nm (60 .ANG.) thick and two InGaAsP barrier layers 10 nm (100 .ANG.) thick, having an energy band gap corresponding to a light wavelength of 1.31 .mu.m, laminated alternatingly, and the light confinement layers 130 and 150 comprise InGaAsP having a composition with an energy band gap corresponding to a wavelength of 1.31 .mu.m and a thickness of 68 nm (680 .ANG.) is 1.8%. This light confinement coefficient is significantly improved relative to a structure without light confinement layers. In addition, the electric field distribution has a narrow peak in the active layer, as shown in FIG. 18(b).
The results show that the performance of the semiconductor laser having an active layer comprising an MQW structure is only insignificantly improved by introducing a multi-quantum well layer into the active layer and that a significant improvement was realized for the first time by inserting a light confinement layer.
In order to lower the threshold current of a semiconductor laser, it is necessary to confine the carriers to the active layer effectively. Particularly, in a quantum well semiconductor laser having an active layer with quite a small thickness, below about 10 nm (100 .ANG.), the carriers injected into the active layer flow out to the cladding layer and are not effectively utilized. In order to lower the threshold current of a semiconductor laser by preventing such an overflow of carriers, the structure shown in FIG. 15 has been proposed. FIG. 15 is a cross-sectional view showing a conventional MQW semiconductor laser structure comprising InGaAsP series materials. In FIG. 14, reference numeral 201 designates an n type GaAs substrate. An n type In.sub.0.5 (Ga.sub.0.3 Al.sub.0.7).sub.0.5 P cladding layer 202 is disposed on the n type GaAs substrate 201. An In.sub.0.5 (Ga.sub.0.5 Al.sub.0.5).sub.0.5 P guide layer 203 is disposed on the p type cladding layer 202. An MQW active layer 204 comprising a plurality of InGaP well layers and a plurality of In.sub.0.5 (Ga.sub.0.5 Al.sub.0.5).sub.0.5 P barrier layer alternatingly laminated with each other is disposed on the guide layer 203. An In.sub.0.5 (Ga.sub.0.5 Al.sub.0.7).sub.0.5 P guide layer 205 is disposed on the MQW active layer 204. A multi-quantum barrier (MQB) structure 206 comprises InGaP layers and In.sub.0.5 (Ga.sub.0.3 Al.sub.0.7).sub.0.5 P layers alternatingly laminated with each other and an In.sub.0.5 (Ga.sub.0.3 Al.sub.0.7).sub.0.5 P upper cladding layer 207 is disposed on the MQB structure 206. A p type InGaAsP cap layer 208 is disposed on the p type upper cladding layer 207. Reference numeral 209 designates a current blocking layer, reference numeral 210 designates a p type GaAs layer, reference numeral 211 designates an n side electrode, and reference numeral 212 designates a p side electrode.
FIG. 16 is a diagram showing the energy band structure of the conduction band edge in the vicinity of the active layer of the semiconductor laser shown in FIG. 15. In FIG. 16, the same reference numerals designate the same or corresponding portions as shown in FIG. 15.
The MQB structure includes a laminated plurality of heterojunctions produced by mutually different composition semiconductor layers, each of which is several atomic layers thick. The first person to introduce an MQB structure into a semiconductor laser was Professor Iga of Tokyo Institute of Technology, and that structure was described, for example, in Japanese Published Patent Application 63-46788. This MQB structure includes a laminated plurality of GainAsP thin films and a plurality of InP thin layers between bulk crystal active and cladding layers supplying the flow of carriers from the active layer to the cladding layer in high temperature operation of a semiconductor laser. Therefore, the temperature characteristic of the laser is improved.
On the other hand, in the quantum well semiconductor laser, the phenomenon of carrier overflow can be seen even in the normal temperature operation and the introduction of the MQB structure lowers the threshold current of the laser at normal temperature operation. In the prior art example of FIG. 15, six InGaAs layers, all 2 nm (20 .ANG.) thick, and six In.sub.0.5 (Ga.sub.0.3 Al.sub.0.7).sub.0.5 P layers, respectively 50 nm (500 .ANG.) thick, 6 nm (60 .ANG.) thick, 3 nm (30 .ANG.) thick, 3 nm (30 .ANG.) thick, 3 nm (30 .ANG.) thick, and 3 nm (30 .ANG.) thick, from the side in contact with the guide layer 205 are alternatingly laminated. When such short period potential barriers exist, the electrons exhibit wave behavior and an interference effect also arises in an appropriately designed structure. Therefore, the electrons react to an energy barrier larger than the potential barrier of the actual material and are reflected. In other words, the electrons flowing out from the active layer are reflected by the MQB structure and are returned to the region of the guide layer. In the figure, the increment of the energy barrier reacted to is shown as .DELTA.Ue added to the band-discontinuity in the conduction band.
In this way, because the prior art quantum well semiconductor laser shown in FIG. 15 has a structure in which an MQB structure is provided between the guide layer (light confinement layer) and the cladding layer, the overflow of electrons from the active layer to the cladding layer is suppressed and the electrons are effectively confined to the active layer whereby the threshold current of the semiconductor laser is reduced.
As described above, the introduction of the light confinement layer is indispensable to a semiconductor laser having an active layer with a quantum well structure. However, because the light confinement layer is designed to have an intermediate composition between the well layer and the cladding layer of the quantum well active layer, the potential barrier to electrons injected into the well layer is restricted by the light confinement layer. Therefore, the electrons are likely to overflow to the barrier layer or light confinement layer of the quantum well active layer, causing the threshold current of the laser to increase and the dynamic characteristics of the laser to deteriorate significantly. This is actually pointed out by W. Rideout et al, IEEE Photon Technology Letters, Volume 3, pages 784-786, 1991.
In the prior art quantum well semiconductor laser shown in FIG. 15, the overflow of electrons into the cladding layer is suppressed but no consideration is given to preventing the overflow of electrons into the barrier layer and the light confinement layer of the quantum well active layer and there also arise the above-described problems in this prior art device.